Developing tissues rely on the integration of several biochemical pathways to regulate cell proliferation and differentiation. The cell surface receptors EGFR and Notch both contribute to these processes and precise regulation of these signaling pathways is critical for tissues to produce both the correct cell number and cell types. In the developing retinas of both vertebrates and invertebrates, EGFR and Notch control proliferation of retinal precursors and differentiation of photoreceptors and support cells. The strength and duration of their signaling relies upon not only the presence of activating ligands, but also factors that regulate their internalization from the plasma membrane and degradation in the lysosome. We have found that the cell adhesion molecule Echinoid (Ed) regulates EGFR and Notch signaling in the developing Drosophila retina. Mutations in ed cause over-proliferation of cells in the retina and formation of many extra R8 cells, the first photoreceptor type in the eye. Consistent with these phenotypes, retinal tissue lacking ed contains elevated levels EGFR, suggesting that Ed normally promotes its degradation. In addition, the Notch ligand Delta appears mislocalized in ed mutant tissue, suggesting that Ed also modulates its subcellular localization. This proposal seeks to elucidate the mechanism by which Ed regulates Notch and EGFR signaling Drosophila eye development. Through a variety of genetic, molecular, and biochemical approaches, this proposal aims to examine how loss of ed alters EGFR and Delta localization, to pinpoint the domains of Ed important for its roles in EGFR internalization and photoreceptor development, and to identify proteins that Ed binds to directly. Although this work will be done using Drosophila, a genetically-tractable organism, it is also applicable to human health. Ed is thought to be a functional homolog of the nectins, a family of vertebrate cell adhesion molecules that regulate a variety of developmental processes. These include proliferation, contact inhibition, and protein internalization. In addition, the developmental programs leading to differentiation of R8 photoreceptors in Drosophila are very similar to those leading to retinal ganglion cells in vertebrate eyes, suggesting that the data collected here will inform our understanding of both invertebrate and vertebrate retinal development. These experiments will further our understanding of the roles of cell adhesion molecules in modulating signaling pathways, and of the cues that drive retinal differentiation. PUBLIC HEALTH RELEVANCE: This proposal examines the activity of the protein Echinoid, which regulates both the number and type of photoreceptors present in the Drosophila retina. The biochemical pathways that influence photoreceptor formation in Drosophila and humans are very similar. Thus, this work will improve scientists'understanding of how photoreceptors form, and will further the goal of someday being able to replace photoreceptors in people with retinal degeneration.